Petroleum prospecting method



United States Patent PETROLEUM PROSPECTING METHOD William D. Rosenfeld, Fullerton, Calif., assignor to California Research Corporation, San Francisco, Calif., a corporation of Delaware No Drawing. Application September 28, 1956 Serial No. 612,574

7 Claims. (Cl. 195-1035) This invention relates to methods of prospecting for subterranean petroleum deposits and relates more particularly to bacteriological methods for locating underground petroleum accumulations.

There has, for a considerable number of years, been extensive activity in developing methods for locating oil and gas producing formations by analyzing various materials from the earth for the presence of various compounds or substances. Such methods have included the analysis of soil samples for the concentrations therein of selected light hydrocarbons such as ethane and methane, and the analysis of surface soil samples for concentrations therein of aerobic, hydrocarbon-consuming bacteria. It has been generally considered that a relatively high content of hydrocarbons or bacteria in the surface soil sample indicated that hydrocarbons had escaped from some accumulation in a subterranean oil or gas producing formation in the vicinity and had found its way through underground channels to the point where the sample or samples were obtained.

However, none of these methods have been particularly successful in petroleum exploration, owing to the uncertainties inherent in the assumptions involved in the methods, the difliculty of collecting uncontaminated samples, the possibility of bacterial action destroying the hydrocarbon present in the samples, or creating new ones, and the difliculty of accurately analyzing the collected samples for hydrocarbon or bacteria content. For example, gaseous hydrocarbons may originate in other than liquid oils, so that their presence in samples in even anomalously high concentrations is not necessarily indicative of the proximity of accumulations of liquid hydrocarbons. Similarly, the analysis of soil samples for bacteria has the disadvantage that such samples are subject to contamination by bacteria of surface origin which have no connection with the hydrocarbons sought. Addiionally, a given surface soil sample necessarily represents only a relatively small area or volume, so that a large number of such samples must be taken to obtain a representative sampling of an area.

Broadly, the present invention is directed toward the concept of analyzing samples of waters from the earth for the presence therein of bacteria capable of oxidizing selected liquid hydrocarbons. One prior method of geochemical prospecting involves analyzing waters from the earth for the concentrations therein of liquid hydrocarbons. The hydrocarbons which are utilized in such method are the liquid hydrocarbons having from about '5 to about 11 carbon atoms and include parafiinic, cycloparafiinic and-aromatic compounds, such as pentane, hexane, decane, cyclohexane, dimethyl cyclopentane, phenylcyclohexane, benzene, toluene, xylene, and ethyl benzene. The presence of these liquid hydrocarbon compounds in the sampled waters in sufficiently high concentrations is believed to be a reliable indication of the proximity of underground petroleum accumulations. The mechanism or mechanisms by which these light hydrocarbon compounds reach the surface or near-surface are not known with certainty, but it appears that diifusion, the natural buoyancy of the hydrocarbons in water, and a compaction of sediments, with a resultant forcing out of the water therein, are the most likely causes.

The present invention is based on a correlation between the presence in water samples of selected liquid hydrocarbon compounds, including oxidized hydrocarbon compounds, as an indication of the possible proximity of liquid hydrocarbon. accumulations, and the presence of hydrocarbon-consuming or oxidizing bacteria in these Waters as a similar and analogous indication of the possible proximity of such hydrocarbon accumulations. The types of bacteria sought in the present invention include the so-called nitrate-reducing type, such as the genus Pseudomonas, which are primarily aerobic and which appear to be capable of oxidizing all types of hydrocarbons and oxidized hydrocarbons. The bacteria sought also include the anaerobic sulfate reducing bacteria, such as the genus Desulfovibrio, which oxidize primarily the paraffinic hydrocarbons containing more than 10 carbon atoms, as well as the oxidation products of such hydrocarbon's. Preferably, a given water sample is analyzed for the presence of both of the above types of bacteria, although it will be understood that the presence of either type of bacteria alone may be utilized as an indication of the possible presence of liquid hydrocarbon accumulations.

The sampled waters utilized in the present invention are preferably obtained from formations at depth or from ground waters. In the following descriptions the terms ground waters and formation waters are understood to mean the following: Ground waters are those waters which are below the water table and which are generally fresh and can be utilized for domestic water supplies. Formation waters are those which are usually deeper than ground waters, are commonly vmore saline, and may occur in oil-bearing or potentially oil-bearing strata. Waters above the water table are preferably not utilized, as they may contain aerobic bacteria in such large numbers that the hydrocarbon molecules present in the water are substantially oxidized to thus destroy the material sought.

In general the present invention comprises the steps of collecting the samples of ground water or formation water, the analysis of these samples in some manner to determine the concentrations of the selected hydrocarbonconsuming or oxidizing bacteria therein, and the correlation of the concentrations so determined with the sample locations to determine anomalously high values of such concentrations indicative of the possible presence of hydrocarbon accumulations.

In carrying out the present invention, samples of the water to be analyzed may be obtained fromv any suitable source in the area to be prospected. As indicated above, the samples are preferably taken from waters below the water table to avoid the large numbers of aerobic bacteria usually present in surface and near-surface waters, and to avoid possible contamination of the samples. Nearsurface waters may be sampled in existing water wells or at surface springs or seeps, or by drilling relatively shallow holes to tap such waters. In the case of formation waters at depth, samples may be obtained by drilling, usually in conjunction with the normal drilling operations of a conventional well. The size of a sample obtained may be varied, but I have found that a onepint sample representsa suitable compromise between the considerations of ease of sample handling and the need for a large enough sample to obtain a representative distribution of bacteria.

After collection, the Water samples are analyzed by any suitable known means to determine the concentrations therein of the hydrocarbon-oxidizing bacteria. Such or the measurement of oxygen uptake by the bacteria in the sample, as is well known in the bacteriological art. However, these methods are laborious and time-consuming procedures, and therefore I prefer to employ a novel nitrate-reduction or sulfate-reduction method to obtain a rapid and simple determination of bacterial count. For nitrate-reducing bacteria, this method is based on a correlation between the ability of the nitrate-reducing bacteria to oxidize or consume hydrocarbons and the ability of these same bacteria to reduce a nitrate compound to a nitrite compound, so that a measurement of the extent of the reduction of nitrate to nitrite in a water sample is a measure of the bacterial count in the sample and hence a measure of the amount of liquid hydrocarbons present in the sample. Similarly, for sulfate-reducing bacteria, the correlation between the ability of these sulfate-reducing bacteria to reduce sulfates to sulfides and the ability of these bacteria to oxidize hydrocarbons or oxidized hydrocarbons is utilized to obtain an indication of the amount of liquid hydrocarbons in the sample.

The details of practicing the present invention in one representative form are as follows. After collection of the water samples as indicated above, the samples are combined with a suitable bacteria-culture medium and the resulting solution is allowed to incubate at a suitable temperature and for a suitable length of time. At the end of the incubation period the solution is analyzed to provide a measure of the concentration of bacteria in the sample. Such analysis may be on any suitable basis, such as by the addition to the culture solution of a reagent for producing a color change in the culture solution in accordance with the nitrite or sulfide concentration therein, and the analysis of this solution with a colorimeter to obtain a quantitative measure of the nitrite or sulfide concentration and hence a measure of the bacteria present in the sample. Alternatively, a novel dilution and color analysis procedure, to be discussed in detail below, may be utilized to provide a simple and rapid method of determining the bacterial concentration.

Since the bacteria concentration in each sample corresponds to the concentration of selected liquid hydrocarbons in the sample, a correlation of the bacteria concentrations with the sample locations enables the determination of areas in which anomalously high values of liquid hydrocarbons are present in the waters, thus indicating favorable conditions for the discovery of accumulations of gas and oil deposits.

The composition of the culture medium utilized in the present invention may be varied widely, as is well known to bacteriologists, and an example of a culture medium which gives excellent results in the present prospecting method when the nitrate-reducing bacteria are sought is the following Table I.Basal medium FeSO "grams-.. 0.1 K HPO do 0.5 KNO do 1.0 Synthetic sea water (400 percent) ml 25 Distilled water ml 975 The reaction of the mixture is adjusted to pH 7.8, and 2.0 grams of agar are added. The mixture is boiled to dissolve the agar and then allowed'to cool.

A representative synthetic sea water for use in the basal medium of Table I may be as follows:

Table lI.-Synthetic sea water (400 percent) A. Distilled .water ml 2500 NaCl grams 778 B. Distilled water ..ml 3500 Na SO -1 grams 130 C. Distilled water ml 1000 MgCl -6H O grams 352 4 a D. Distilled water m1 1000 KCI grams 11 NaHCO do 3.2 KBr do 1.6 SrCl 6H O do .67 H3BO3 d0 .43 Na SiO 9H O do .08 NaF do .05 NH NO do .03 FePO -4H O do .02

Components A, B, C and D of Table II are dissolved and 25 ml. of this mixture is combined with the other components of Table I to form the basal medium. After preparation of the basal medium in accordance with the above procedure, 50 ml. of a hydrocarbon emulsion is added thereto. The hydrocarbon emulsion is added to the mixture to enable the bacteria in the water sample to multiply rapidly enough so that a count or measure of the bacteria may be obtained within a reasonable length of time. A representative hydrocarbon emulsion may be as follows:

Table lI1.-Hydr0carbon emulsion Paratfin or mineral oil (or a pure hydrocarbon such as hexadecane) rnl 5.0 Distilled water ml 45.0 Gum arabic grams 0.1

These components are emulsified in a mechanical agitator and then passed through a homogenizer. The completed nutrient medium is placed in test tubes and the tubes are sterilized in an autoclave or pressure sterilizer at 121 C. for 15 minutes.

After preparation of this culture medium, the sampled water is added to the culture medium and the bacterial concentration is determined, preferably by the following novel dilution and color analysis procedure. To nine milliliters of the prepared culture medium in a first test tube is added one milliliter of the water sample, producing a dilution of the ground Water sample in the total solution in the proportion of one in 10. One ml. of this solution from the first test tube is then placed in a second test tube which contains 9 ml. of prepared culture medium, and these components are mixed in the second test tube to produce a dilution of the water sample in the total solution of one in 100. Similarly 1 ml. of the mixture from the second test tube is placed in a third test tube which contains 9 ml. of prepared culture medium, and the components mixed to produce a dilution of the water sample in the total solution of 121000. This dilution is then carried on any desired number of times to produce a series of 9 ml. mixtures of culture medium and'water sample, the dilution of the water sample in the total solution of each test tube progressively varying from tube to tube by increasing powers of 10.

After the above dilutions, the test tubes are incubated at a suitable temperature, such as from 25 -30 C., until there is no further change in the maximum dilution that reduces nitrate to nitrite. To test for the reduction of nitrate to nitrite, any suitable procedure may be used, such as the admixture in a spot plate depression of approximately 0.5 ml. of the solution from the test tube with 2 drops of sulfanilic acid solution plus 2 drops of wnaphthylamine solution, followed by visual inspection for color change from colorless to pink or red. I have found that this test for the reduction of nitrate to nitrite is very easily performed and provides an accurate indication of the reduction of nitrate to nitrite.

Where the above set forth dilution testing procedure is utilized, the bacterial count of a given water sample may be expressed as the reciprocal of the dilution of the most dilute solution which shows bacterial growth. For example, if a given Water sample showed bacterial growth up to dilutions of one part in 1,000,000, but showed no growth for a dilution of 1 part in 10,000,000, the bacterial count could be expressed as a reciprocal of'this dilution, or 1,000,000 organisms per cubic centimeter. These bacterial counts may then be correlated in some suitable manner with the sample locations, such as by contour mapping of the locations of isoconcentrations of bacterial count on a map ofthe prospected area.

While in the above dilution analysis procedure it was assumed that the dilution progressively increased by powers of 10, it will be understood that the difference between successive dilutions can be decreased to any suitable number, if more exact information is desired as to the bacterial count of a given water sample. Similarly, although the above dilution analysis procedure does not provide an exact measure of the bacterial count, it is believed that the speed and'simplicity with which this analysis procedure may be performed renders it very suitable for the analysis of large numbers of water samples in a prospecting area and will permit the use of the method of the present invention in surveying large areas.

The analysis method outlined above for determining the concentrations of nitrate-reducing bacteria may be utilized, with suitable modifications, in determining the concentration of sulfate-reducing bacteria in water sarnples. For sulfate-reducing bacteria, a very satisfactory culture medium is as follows:

Table I V.Culture medium for sulfate-reducing bacteria Culture medium:

FeSO -(NH SO -6H O grams 0.1 Ascorbic acid do .2 K2HPO4 do...... -5 (NH4)2SO4 dO Calcium lactate do 3.5 400% synthetic sea water ml 25 H O ml 975 The synthetic sea water of Table IV may be identical to that whose composition is listed in Table III. The components listed in Table IV are combined and the pH thereof is adjusted to 7.8. Three grams of agar are then added and the'mixture is boiled. After cooling, the culture medium is placed in test tubes and sterilized.

After sterilization, the water samples are added to the culture medium, preferably utilizing the novel dilution procedure set forth above for the nitrate-reducing bacteria to produce successively more dilute solutions of the sampled water in the culture medium. After incubation of the water and culture medium solutions for a suitable length of time and at a suitable temperature, an analysis is made for bacterial concentration. In the case of sulfate-reducing bacteria, a reliable indication of bacterial growth is the production of hydrogen sulfide, with the consequent blackening of the culture medium. Thus, in the progressive dilution procedure for sulfate-reducing bacteria, bacterial growth could be determined by selecting the maximum dilution which was blackened by the production of hydrogen sulfide, indicating bacterial growth, and utilizing the maximum dilution, or its reciprocal, as a measure of bacterial count in that particular water sample. The different values of bacterial count may then be correlated with the sample locations, such as by plotting on a map contours of bacterial isoconcentrations, the areas of high bacterial concentration indicating areas favorable for the accumulation of oil and gas.

The following table illustrates the operation of the present invention in prospecting in an area having known oil deposits. The water samples were obtained in and near known oil fields at the distances indicated, and the bacterial concentrations are indicated by a number representing the exponent of ten corresponding to the maximum dilution showing bacterial growth, as indicated above in describing the analysis procedure. Thus, the number 4 for a given sample indicates that a dilution of a 6 one in ten thousand was the maximum dilution at which that sample showed bacterial growth.

Table V Lateral Oil Distance Bacterial Sample Field of Sample Concen-.

from Oil tration Field, Miles 1 0 7 1 )4 3 1 at 2 1 2 4 2 1 9t 3 2 0 5 2 $4 6 2 1% 2 2 1% 3 2 1% 2 2 1% 2 It will be seen from Table V that the bacterial concentrations in the sampled waters do appear to correlate with the accumulations of petroleum, the concentrations of the bacteria, in general, increasing as the oil accumulations are approached.

Although but a few embodiments of the present invention have been described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention or the scope of the appended claims.

I claim:

1. The method of detecting subterranean deposits of petroleum comprising the steps of obtaining samples of water from the earth in the area to be investigated, analyzing said samples to determine the concentrations therein of bacteria selected from the group consisting of nitrate-reducing, hydrocarbon-oxidizing bacteria and sulfate-reducing, hydrocarbon-oxidizing bacteria, and correlating the concentrations so determined with the 10- cations of said samples to determine anomalously high values of said concentrations indicative of the proximity of an underground petroleum accumulation.

2. The method of detecting subterranean deposits of petroleum comprising the steps of obtaining samples of water from the earth in the area to be investigated, analyzing said samples to determine the concentrations therein of nitrate-reducing, hydrocarbon-oxidizing bacteria, and correlating the concentrations so determined with the locations of said samples to determine anomalously high values of said concentrations indicative of the proximity of an underground petroleum accumulation.

3. The method of detecting subterranean deposits of petroleum comprising the steps of obtaining samples of water from the earth in the area to be investigated, analyzing said samples to determine the concentrations therein of sulfate-reducing, hydrocarbon-oxidizing bacteria, and correlating the concentrations so determined with the locations of said samples to determine anomalously high values of said concentrations indicative of the proximity of an underground petroleum accumulation.

4. The method of detecting subterranean deposits of petroleum comprising the steps of obtaining samples of ground waters from the earth in the area to be investigated, analyzing said samples to determine the concentrations therein of bacteria selected from the group consisting of nitrate-reducing, hydrocarbon-oxidizing bacteria and sulfate-reducing, hydrocarbon-oxidizing bacteria, and correlating the concentrations so determined with the locations of said samples to determine anomalously high values of said concentrations indicative of the proximity of an underground petroleum accumulation.

5. The method of detecting subterranean deposits of petroleum comprising the steps of obtaining samples of formation water from the earth in the area to be investigated, analyzing said samples to determine the concentrations therein of bacteria selected from the group consisting of nitrate-reducing, hydrocarbon-oxidizing bacteria and sulfate-reducing, hydrocarbon-oxidizing bacteria, and correlating the concentrations so determined with the 10- cations of said samples to determine anomalously high values of said concentrations indicative of the proximity of an underground petroleum accumulation.

6. The method of detecting subterranean deposits of petroleum comprising the steps of obtaining samples of ground waters from the earth in the area to be investigated, analyzing said samples to determine the concentrations therein of nitratereducing, hydrocarbon-oxidizing bacteria, and correlating the concentrations so determined with the locations of said samples to determine -anomalously high values of said concentrations indicative of the proximity of an underground petroleum accumulation.

7. The method of detecting subterranean deposits of petroleum comprising the steps of obtaining samples of References Cited in the file of this patent UNITED STATES PATENTS 2,294,425 Sanderson Sept. 1, 1942 OTHER REFERENCES Zinssers Textbook of Bacteriology, by Smith et al.

' 9th ed., '1948, publ. by Appleton-Cen-tury-Crofts, Inc.

(N.Y.), pages 57 and 58. 

1. THE METHOD OF DETECTING SUBSTERREAN DEPOSITS OF PETROLEUM COMPRISING THE STEPS OF OBTAINING SAMPLES OF WATER FROM THE EARTH IN THE AREA TO BE INVESTIGATED ANALYZING SAID SAMPLES TO DETERMINE THE CONCENTRATIONS THEREIN OF BACTERIA SELECTED FROM THE GROUP CONSISTING OF NITRAE-REDUCING, HYDROCARBON-OXIDIZING BACTERIA AND SULFATE-REDUCING, HYDROICARBON-OXIDIZING BACTERIA, AND CORRELATING THE CONCENTRATIONS SO DETERMINED WITH THE LOCATIONS OF SAID SAMPLES TO DETERMINE ANOMALOUSLY HIGH VALUES OF SAID CONCENTRATIONS INDICATIVE OF THE PROXIMITY OF AN UNDERGROUND PETROLEUM ACCUMULATION. 